This unique signature can also be used as unique encryption key specific to the circuit. In this case, the storage of the key within the integrated circuit is not required.
The signatures are generated directly by the circuits. Human intervention not being required, the resistance to attacks, notably of observation attack type, is improved.
There exist in the prior art various ways of implementing PUF functions. Thus, the article by R. Pappu entitled Physical One-Way Functions, PhD Thesis, Massachusetts Institute of Technology, March 2001, describes what constitutes an optical PUF. Optical PUFs are composed of a transparent material comprising randomly dispersed particles allowing the deviation of laser light.
Coating PUFs are also used. This type of PUF is described in the article by P. Tuyls, B. Skoric and T. Kevenaar entitled Security with Noisy Data: Private Biometrics, Secure Key Storage and Anti-Counterfeiting, Secaucus, N.J. USA: Springer-Verlag New York, 2007. In this case, an opaque material is randomly doped with dielectric particles and is positioned above the integrated circuit.
A family of PUFs called silicon PUFs uses the structural incoherencies introduced by methods for fabricating integrated circuits. The difference in dispersion between the wires and the transistors making up said circuits is indeed significant from one circuit to another, even if they form part of the same slice. This family comprises notably arbiter PUFs, ring oscillator PUFs and SRAM PUFs. Silicon PUFs may be implemented in ASIC or FPGA circuits without any technological modification.
Arbiter PUFs are described in the article by B. Gassend, D. E. Clarke, M. van Dijk, and S. Devadas, entitled Silicon physical random functions, ACM Conference on Computer and Communications Security, 2002, pages 148-160. In this type of PUF, one and the same signal propagates by following two paths of a delay circuit, the two circuits being distinct and being configurable with the aid of control words. An arbiter compares the delay between the two signals resulting from these two propagations, and the result of this comparison culminates in the signature of the integrated circuit. One of the drawbacks of this type of PUF is that the elements allowing the parametrization of the paths must be balanced in terms of delays, thereby rendering their design difficult.
PUFs with pairs of ring oscillators are also silicon PUFs. They are described in the article by G. E. Suh and S. Devadas entitled Physical unclonable functions for device authentication and secret key generation, DAC, 2007, pages 9-14. The frequencies generated by a pair of identical ring oscillators are compared. The result of this comparison culminates in the signature of the integrated circuit. A drawback of ring oscillators is that said oscillators are sensitive to so-called second-order effects such as for example the effects related to the mutual coupling between the oscillators or to the disturbances introduced on an oscillator during an attack.